Chemical conversion treatment is a type of treatment in which the surface of a steel material and a corrosive solution are chemically reacted with each other to form a film of a corrosion product having good adhesion to the surface of the steel material. Chemical conversion treatment is frequently referred to as phosphate treatment, chromate treatment, oxalate treatment, etc. depending upon the type of corrosive solution which is employed.
However, a chemical conversion film cannot be formed by such conventional chemical conversion treatment on a steel such as a high-Cr steel.
As disclosed in Japanese Published Unexamined Patent Application 57-82478, for example, a method is known in which chemical conversion treatment is performed on the surface of a steel material using a chemical conversion treatment liquid based on an alkali metal phosphate and containing a titanium compound and a chlorate salt, after which further chemical conversion treatment is performed using a chemical conversion treatment liquid containing a zinc phosphate. However, this method has the disadvantage that treatment must be performed two times. Moreover, this method cannot form a sound chemical conversion film of a phosphate on a high-Cr steel such as a 13% Cr steel.
Japanese Published Unexamined Patent Application 5-40034 discloses a method of surface treatment using a chemical conversion treatment liquid containing manganese and phosphoric acid to which fluoride ions are added. However, even with this method, it is not possible to form a chemical conversion film on a Cr-containing steel.
Oil well steel pipes are connected to each other through couplings. For this purpose, male threads formed on the ends of oil well steel pipes are mated with female threads formed on the inner surface of couplings, and the threads are tightened to form a gas- and liquid-tight joint connecting the pipes. At the time of to tightening, a large torque is applied to the threads, so it becomes easy for defects such as galling to occur on the thread surface, which reduces the number of times which the oil well steel pipes can be repeatedly connected to each other. In addition, if corrosion occurs on the thread surface, it becomes difficult to guarantee an adequate gas tightness and liquid tightness.
Accordingly, in the past, the thread surface of a threaded joint for oil well steel pipes made of a Cr-containing steel was plated with a soft metal such as Cu in order to prevent galling. However, due to the man hours required for plating, the plating method is not satisfactory, and there is room for improvements.